CURLOPT_WRITEDATA (3) - Linux Man Pages
CURLOPT_WRITEDATA: custom pointer passed to the write callback
NAMECURLOPT_WRITEDATA - custom pointer passed to the write callback
DESCRIPTIONA data pointer to pass to the write callback. If you use the CURLOPT_WRITEFUNCTION(3) option, this is the pointer you'll get in that callback's 4th argument. If you don't use a write callback, you must make pointer a 'FILE *' (cast to 'void *') as libcurl will pass this to fwrite(3) when writing data.
The internal CURLOPT_WRITEFUNCTION(3) will write the data to the FILE * given with this option, or to stdout if this option hasn't been set.
If you're using libcurl as a win32 DLL, you MUST use the CURLOPT_WRITEFUNCTION(3) if you set this option or you will experience crashes.